During operation, a gas turbine engine compresses intake air, mixes the compressed air with fuel, and ignites the fuel-air mixture to produce combustive gasses, which are then expanded through a number of air turbines to drive rotation of the turbines and produce power. Significant quantities of energy are expended to compress the intake air before the hot, compressed air is supplied to the engine's combustion chamber for fuel injection and combustion. Leakage of the compressed air from the compressor section, especially from the downstream end of the high pressure compressor stage, results in a direct penalty against the engine's power-to-weight ratio and overall fuel efficiency. For this reason, air-to-air shaft seals are commonly positioned around the engine spools or shafts to reduce the leakage of pressurized airflow from the high pressure compressor section and other such high pressure zones of the engine to neighboring areas of the engine containing lower air pressures. Traditionally, labyrinth seals have been widely utilized in view of their relative simplicity and low cost. In one common design, the labyrinth seal includes a plurality of rotating disks or knives, which are affixed to the shaft and which are separated from a neighboring stationary structure (e.g., a land) by a small axial gap to provide a relatively low, predictable leakage through the seal during engine operation.
While providing a relatively low initial leakage, the integrity of a labyrinth seal can degrade over time due to intermittent contact between the spinning disks of the labyrinth seal and neighboring static components. Contact between spinning and static components may occur due to radial displacement of the shaft as the engine transitions through critical modes and/or as radial impulse forces are imparted to rotor assembly during aircraft maneuvering, hard touchdown, and the like. As an alternative to labyrinth seals, finger seals have been developed that include a plurality of elongated fingers, which extend radially inward from a static structure to contact and form an annular seal around the shaft. The fingers are radially resilient and thus able to deflect to accommodate radial displacement of the shaft. Advantageously, finger seals typically provide superior sealing performance as compared to labyrinth seals; however, finger seals are also subject to wear due to their contacting design and may require a lengthy break-in process to ensure proper seal operation. Additionally, the constant rubbing between the fingers and the rapidly spinning shaft may generate significant quantities of heat, which can potentially damage the finger seal or the shaft. It has been suggested that relatively large, axially-extending pads may be joined to the inner terminal ends of the fingers to promote seal lift-off during high speed rotation of the shaft. However, such axially-elongated finger pads tend to be relatively poor at achieving and sustaining seal lift-off due, at least in part, to the formation of divergent wedges between the inner surfaces of the finger pads and the outer surface of the rotating shaft. Furthermore, even when able to achieve and sustain seal lift-off, such finger pads tend to impart undesirable twisting forces to the torsionally-soft fingers thus interfering with proper operation of the seal.
It would thus be desirable to provide embodiments of a lift-off finger seal suitable for forming a low leakage annular seal around a shaft within a gas turbine engine or other rotating machine, which does not impart undesirable twisting forces to the seal fingers and which reliably achieves seal lift-off during high speed rotation of a shaft to reduce seal wear, to prolong seal life, and to avoid the need for a prolonged seal break-in process. Ideally, embodiments of such a lift-off finger seal would also be radially- and axially-compact, lightweight, and relatively inexpensive to manufacture. It would also be desirable to provide embodiments of a gas turbine engine including such a lift-off finger seal, as well as embodiments of a method for manufacturing such a finger seal. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent Detailed Description and the appended Claims, taken in conjunction with the accompanying drawings and the foregoing Background.